Well... I can only share some of my experience.
I am happy with this lens.
It is very comfortable to hold
Focusing is fast and the IS if fantastic (up to 3 f-stop handheld)
The weight is light about the same as 75-300IS.
I got this lens was to replace my 75-300IS.
Why I chose this lens is because it is light, compact and long focal length with IS.
Some said the price is expensive? ....because this is not a L lens. The price I paid was equivalent to a 70-200L f4 + 1.4x TC...but IMO for the size, weight and quality achieve with the new IS I think I can justify myself..I am paying for a new technology of is own class.
Here is some pictures from this lens.
http://www.pbase.com/kcchuan/in_box
http://www.pbase.com/kcchuan/bishan_park&page=all
Alvin